1. Field of the Invention
The present invention relates to a liquid crystal display and more specifically, to an active-matrix liquid crystal display driven by a thin film transistor.
2. Description of the Background Art
In recent years, FPDs (flat panel displays) such as liquid crystal panels have been used in many fields such as televisions, car navigation systems and computers for reasons such as lightweight and thin properties and low power consumption. These FPDs are requested to achieve higher display qualities year after year, and employing a driving system suitable for increasing a contrast and a viewing angle has become a mainstream tendency.
In particular, an IPS (in-plane switching) driving system is suitable for satisfying the aforementioned quality requirements, and has been employed by an increasing number of liquid crystal manufacturers. However, the IPS driving system is not an efficient system in terms of manufacture, so that increasing yield in manufacturing factories is an absolute necessity.
In a general liquid crystal display of the IPS driving system, substrates in a pair facing each other are spaced a constant distance by a spacer, and liquid crystal fills in the space between the substrates. One of the substrates is an active-matrix substrate (TFT substrate) including source lines and gate lines arranged to cross each other in a grid pattern, and thin film transistors arranged at the intersections. A pixel electrode and a common electrode are formed on the same substrate, and electric charges accumulated between these electrodes control driving of the liquid crystal. Generally, a transparent film made of a material such as ITO (indium tin oxide) and IZO (indium zinc oxide) is used as a conductive film.
The other substrate is a color filter (CF) substrate including a black matrix (hereinafter called BM) to shield a domain region of a liquid crystal, a color layer, an organic film layer, and a columnar spacer. The CF substrate does not include a conductive film.
The IPS driving system achieves increase in a viewing angle by forming the pixel electrode and the common electrode on the same substrate as described above, and generating an electric field (lateral electric field) in a direction parallel with the substrate, thereby moving liquid crystal molecules laterally. Hence, liquid crystal orientation at the side of the source line is disturbed due to the effect of the electric field generated from the source line.
Japanese Patent Application Laid-Open No. 2003-295207 discloses a technique intended to prevent this problem. According to this technique, a conductive film (common electrode) is formed over the source line via an insulating film. In Japanese Patent Application Laid-Open No. 2003-295207, forming the common electrode over the source line via the insulating film prevents disturbance of the electric field generated from the source line.
However, if a defect (foreign matter or damage) is generated in the insulating film, forming the common electrode over the source line via the insulating film may generate a short between the source line and the common electrode, and this short may be visually recognized as a linear defect.
A dot-like defect may be generated in a liquid crystal panel, and this defect is not conspicuous in many cases. In contrast, a linear defect is generally recognized visually as a defect of a length same as that of the source line and corresponding substantially to the length of one side of a display panel. The aforementioned short, generated even in one place of the source line, is recognized visually as a conspicuous defect, leading to high manufacturing risk. It is important to take action for recovery in response to generation of this problem. Meanwhile, a short generated in any place of the source line becomes a linear defect. Hence, a place of the defect is difficult to specify and action for recovery cannot be taken, resulting in reduction in manufacturing yield. Difficulty in specifying a place of the defect takes a long time to find out a cause of the defect, making diagnosis of the cause a troublesome task.
As described above, in the liquid crystal display of the IPS driving system, a linear defect is generated with a high probability due to a short between the source line and the common electrode, resulting in reduction in manufacturing yield.